How to get an I-94 for a green card?

What is the I-94?
It is the official record of the movements of individuals in and out of the United States (arrivals and departures).

Which family-base immigration forms require I-94 information?  
Form I-130 and form I-485.

Can the I-94 be obtained online?
Yes, most people admitted to the United States after May 2013 were issued an electronic arrival-departure record I-94. Get I-94 

Is there a fee associated with obtaining the I-94 online?
No.

I cannot obtain the I-94 online, what do I do?
The I-94 may be obtained by filing Form I-102, Application for Replacement/Initial Nonimmigrant Arrival-Departure Record, with USCIS.  Note that there is a $445 fee for this service.

Who produces the I-94?
The Department of Homeland Security (DHS) through Customs and Border Patrol (CBP).

Why does the I-94 matter?
An I-94 demonstrates authorized entry into the United States.

What to look for?
Overstays in the United States.